Signature of the atmospheric asymmetries of hot and ultra-hot Jupiters in light curves

Introduction

With the new generation of space telescopes such as the James Webb Space Telescope (JWST), it is now becoming possible to characterize the asymmetries of the atmospheres of exoplanets. The atmospheres of Hot and Ultra-Hot Jupiters are highly heterogeneous and asymmetrical as temperatures of the day-side of tidally-locked planets can be drastically hotter than their respective night-side. The difference between the temperatures on the day-side and the night-side is especially extreme in the case of Ultra-Hot Jupiters and can reach 2000K. It has been shown that these changes may alter analyses and introduce biases into the unraveling of the molecular abundances of species in these atmospheres [1].

Methods

The Pytmosph3R framework [2] is able to generate synthetic observations (transmission or emission spectra), using 1D, 2D or 3D atmospheric models such as Global Circulation Models (GCM). We have extended this tool to introduce time-variability in the model [3], thus enabling us to reproduce light or phase-curves. The model includes thus the rotation of the planet as well as the calculation of the partial coverage of the atmosphere and the planet over the star during the ingress and egress.

Results

We find that the tidally-locked rotation of a Ultra-Hot Jupiter during a transit induces a non-negligible variation of the flux. This variation is a source of information on the chemical and thermal distribution of the atmosphere. We find the day-night thermal gradient present on Ultra-Hot Jupiters has an effect which could be mistaken with stellar limb-darkening: limb-darkening induces a reduction of the intensity of the flux covered by the planet on the edges of the stellar disc compared to the center, while the rotation of the planet and its atmosphere induces a reduction of the projected area of the atmosphere, due to the hotter (and larger) day side being shifted to either the east or the west during the early and late stages of the transit. This is shown in the figure below (the scale of the atmosphere has been exaggerated by a factor of 10 for visual purposes).

At the same time, this signal seems also to produce a signal opposite to tidal deformation. Indeed, the elongated form of the "rugby ball" of a tidally deformed planet covers more area during the early and late stage of the transit.

We also confirm the impact of the atmospheric and chemical distribution on variations of the central transit time, though the variations found are smaller than that of available observational data, which could indicate that the east-west asymmetries are underestimated, due to the chemistry or clouds. The east-west asymmetries being more pronounced in hot Jupiters rather than Ultra-Hot Jupiters, central transit time variations are better observed in lightcurves from Hot Jupiters. We illustrate this with simulations of Wasp-39b (Hot Jupiter) and Wasp-121b (Ultra-Hot Jupiter), such as shown below.

References

[1] W. Pluriel, J. Leconte, V. Parmentier, T. Zingales, A. Falco, F. Selsis, and P. Bordé. Toward a multidimensional analysis of transmission spectroscopy. II. Day-night-induced biases in retrievals from hot to ultrahot Jupiters. Astronomy & Astrophysics, 658:A42, Feb. 2022.

[2] A. Falco, T. Zingales, W. Pluriel, and J. Leconte. Toward a multidimensional analysis of transmission spectroscopy. I. Computation of transmission spectra using a 1D, 2D, or 3D atmosphere structure. Astronomy & Astrophysics, 658:A41, Feb. 2022.

[3] A. Falco, J. Leconte, A. Mechineau, and W. Pluriel. Signature of the atmospheric asymmetries of hot and ultra-hot Jupiters in lightcurves. Astronomy & Astrophysics, arXiv e-prints, arXiv:2402.12355, 2024